Field of the Invention
This invention relates to an electrically non-conductive base for an electrodeless discharge lamp. More particularly this invention relates to an electrically non-conductive base for an electrodeless arc discharge lamp having an outer envelope press sealed over a tubular lamp stem containing a starting aid, wherein the base is secured, in clamshell fashion, to the press seal portion of the lamp and contains means for coupling RF energy to the starting aid for energizing the lamp.
Background of the Disclosure
High intensity electroded arc discharge lamps such as high pressure sodium lamps and metal halide lamps are well known and include a light-transmissive arc discharge chamber or tube hermetically sealed and enclosing within a pair of spaced apart electrodes and a suitable fill such as an inert starting gas and one or more ionizable metals or metal halides. Two of the major causes of lamp failure are sputtering of electrode material onto the lamp envelope and thermal and electrical stresses which result in electrode failure. More recently a new class of high intensity arc discharge lamps has been developed called electrodeless lamps. Such lamps have a light-transmissive, electrodeless arc chamber or tube generally shaped like a pillbox or oblate spheroid and containing a fill which comprises a suitable inert buffer gas and one or more metal halides. Radio frequency (RF) energy applied or coupled to the fill via capacitive or inductive coupling generates a light-emitting arc. In operation of such a lamp via inductive coupling, the arc tube or chamber acts as a single-turn secondary coil of a transformer and is surrounded by an RF energy excitation coil which acts as a primary coil. Various embodiments of such lamps are disclosed, for example, in U.S. Pat. Nos. 4,810,938; 4,959,584; 5,039,903; 5,059,868 and 5,140,227 all of which are assigned to the assignee of the present invention. The electronics required for starting and sustaining the arc discharge in these lamps is costly and complex and also requires the use of a rather large copper or aluminum inductive drive coil to function as the primary coil for carrying the high frequency current required. Hence some development has been directed toward replaceable electrodeless discharge lamps, particularly of the high intensity type employing one or more metal halides as the arc sustaining fill, wherein the lamp itself is physically separate from both the electronics and the inductive drive coil. One such attempt is disclosed in U.S. Pat. No. 4,959,584 as a luminaire having a replaceable electrodeless, HID metal halide arc discharge lamp wherein the arc chamber is cradled in a thermal jacket which in turn is contained within an outer envelope having a conventional metal screw base with a conductive lead passing through the base to the vicinity of the arc tube. U.S. Pat. No. 5,059,868 discloses an HID electrodeless arc discharge lamp wherein the arc chamber is supported in an outer envelope by means of a support rod which contains an electrical inlead for supplying electricity to the arc chamber in order to start the arc. The arc chamber-outer chamber assembly terminates at one end in a conventional metal base having pins for securing in a socket. However, it is preferable to minimize the amount of metal close to the drive coil, because metal (or any conductor) will pick up energy from the coil which will result in heating the metal and loss of system efficiency.